Waterjet propulsion systems are now in widespread use in high speed marine vessels, which are generally defined as those designed to cruise at speeds above 25 knots. A waterjet is essentially a pump that ingests water from underneath the rear of the vessel via a flush mounted intake, and then discharges it at high velocity via a nozzle at the rear of the unit. The reaction to the discharge of this high velocity jet stream provides the thrust to propel the vessel. The power to drive the waterjet pump is typically provided by a gasoline or diesel engine, and in some cases a gas turbine.
Waterjets offer many advantages over conventional propellers and one of particular relevance is the fact that the power absorbed by the waterjet pump is not affected by the speed of the vessel, as is the case with a propeller. With a conventional fixed pitch propeller, the pitch (typically defined as the distance the propeller will progress through the water in one revolution, ignoring slippage) is selected based on the power and rpm of the engine, and the boat speed.
Regardless of the propulsion system type, vessel speed is a function of the load on the vessel and the total power input. With a fixed-pitch propeller that “screws” through the water, if the load increases (for example, with more passengers or cargo on board) and the engine throttle setting and thus power remains constant, the vessel speed drops and the speed of the propeller and engine reduces. This condition results in a higher engine loading. If the vessel load decreases and the engine power remains constant, the vessel speed increases and the speed of the propeller and engine increases. With a diesel engine, this results in the engine over-speeding and a governor will begin to act to restrict this over-speed by reducing the power, thereby limiting the maximum speed at which the vessel may travel at a reduced load.
With a waterjet, the engine cannot be overloaded as the vessel load increases, and similarly cannot over-speed as the vessel load decreases, as the waterjet power absorption characteristic is essentially independent of vessel speed. The waterjet can therefore work efficiently across a broader operating speed range than a propeller.
On a waterjet propelled vessel, a pump impeller must be selected that will absorb the full power of the engine at its rated rpm (revolutions per minute). For example, a typical small diesel engine might deliver 270 kW at 3000 rpm. For a given impeller type, the waterjet power absorption is proportional to the rpm cubed, as follows: P=R×rpm3, where P=the power absorbed at a specified rpm, and R=the impeller rating. For example, if a waterjet is fitted with an impeller that is designed to absorb 10 kilowatts (kW) at 1000 rpm, then at 2000 rpm it will absorb 10×(2000/1000)3=80 kW.
The waterjet power absorption characteristic, being a function of rpm3 and independent of vessel speed, also presents a disadvantage versus propellers. For example, take two identical vessels of the same displacement (weight), engine power and design speed—one fitted with waterjets and the other fitted with propellers. When the vessels are “cruising” at a speed below the maximum speed, the rpm of the propeller will be lower than that of the waterjet due to the aforementioned characteristics of both propulsion systems, even if the engine power being delivered is similar. The waterjet is often perceived to be less efficient due to its higher operating rpm at a particular cruise speed. The higher rpm of the waterjet at cruise may also result in slightly higher noise levels.
By way of example, the graph in FIG. 1 further illustrates the difference between propeller and waterjet propulsion systems with respect to vessel speed versus engine rpm characteristics. FIG. 1 shows the vessel speed versus engine rpm for two identical vessels (36′ Express Cruiser) with the same engine power (twin 440 hp engines), one with waterjets, the other with propellers. As the waterjet is more efficient than the propeller at higher speeds, the waterjet equipped vessel achieves 40 knots, versus 38 knots for the propeller equipped vessel. If these vessels were both cruising at 32 knots, the engines driving the waterjets would be turning at around 2750 rpm, whereas the engines driving the propellers would be turning at around 2550 rpm, which is 200 rpm lower. As the efficiency of the propeller and waterjet is similar at this vessel speed, the engine power delivered in each case would be similar.
In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.
It is an object of the present invention to provide an improved impeller for the pump of a waterjet unit that enables the waterjet unit to operate at an engine speed closer to that of a conventional propeller over a particular vessel speed range, or to at least provide the public with a useful choice.